This invention relates generally to high temperature gas cleanup systems and more particularly to a hot gas clean up multi-membrane filter for filtering out fine dirty particulate matter from a gas and protecting intact filter elements from irreversible blinding operating in the presence of failed filter elements.
Hot gas cleanup systems that are implemented to clean particulate matter from a gas stream are well known in the art. These systems are currently used to clean coal fired gas, pressurized fluidized-bed combustion gas, gasification, and waste incineration. Additionally, these gas cleanup systems may be applied to catalyst and precious metal recovery, calcination, catalytic cracking, and recovering material during chemical processing.
Conventional hot gas cleanup systems generally comprise a filter assembly which is mounted within a pressure vessel. The filter assembly further comprises a plurality of filter elements; a plurality of tube sheets for supporting the filter elements; plenum pipes; and a back pulse system for cleaning particulate matter from the filter elements. These systems may employ a variety of filter elements to filter out dirty particulates from a gas stream.
Examples of typical filter elements employed include cross-flow filters as disclosed in U.S. Pat. No. 4,737,176 to Ciliberti, and hereby incorporated by reference, filter bags as disclosed in U.S. Pat. No. 4,764,190 to Ciliberti and hereby incorporated by reference, porous metal filters, ono-layer membrane filters, ceramic circular cylindrical filters (candle and tube filters) that have either one open end and one closed end, or two open ends, i.e. ASAHI filter element.
Filter elements are generally mounted within a pressure vessel so that a gas can flow through the filter element such that a substantial part of dirty fine particulates within the gas can be removed therefrom. Typically, as a gas stream flows through a filter element, dirty fine particulates collect on the outside of the filter while the clean gas flows through the filter media, into the plenum pipes, and out into the clean gas side of the tube sheets. A substantial amount of the dirty fine particulates that are collected on the outside of the filter element are removed therefrom by a reverse gas pulse provided by the back pulse system which blows the particulates off of the filter element. The removed dirty fine particulates are then collected in a hopper and discharged.
Another type of tubular filter element that is employed is one having side walls that define a bore and two open ends, and a membrane on the inside surface, i.e. ASAHI system. A dirty gas passes along the inside diameter of the tube to be filtered with the cleaned gas passing through the wall and released along the outside diameter of the filter element.
Filter elements may break when particulate ash builds up or bridges, or because of material degradation; i.e. thermal shock, thermal fatigue, creep or cracking growth. When a filter element breaks or fails, particulates continue to travel to the clean gas side of the tube sheet thus causing several problems. Once the particulates are on the clean side, particulates will enter the interior of adjacent intact filter elements when the back pulse system is activated, thereby contaminating the intact filter elements by filling or blinding the filter element pores.
Currently, when filter elements break or fail, the entire cleanup system must be shut down for a number of days so that the broken filter elements can be located and replaced. The contaminated filter elements, however, are left intact. Unfortunately, with the contaminated filter elements still in the system, a loss in the filtering performance of the entire system is incurred. It would, therefore, be desirable to provide an improved filter element that will mitigate the chances of filter contamination and enhance the filtering capabilities of the entire cleanup system.
In accordance with the present invention, a filter element having a plurality of membrane layers is provided. The filter element comprises a filter member having a porous sidewall. The sidewall includes an outer surface and an inner surface which allow a particulate laden gas to flow through so that particulates may be filtered. A first membrane is in communication with the outer surface to prevent particulate matter from penetrating the outer surface. A second membrane is in communication with the inner surface for preventing particulate matter from penetrating the inner surface.